Implement the SSL_CTRL_GET_SHARED_GROUP control

This implements SSL_get_shared_{curve,group}() in a bug-compatible
fashion with OpenSSL.

This is your average OpenSSL-style overloaded parameter API where n >= 0
means "return the n-th shared group's NID" (as if anyone possibly ever
cared about the case n > 0) and n == -1 means "return the number of
shared groups". There is also an undocumented case n == -2 for Suite B
profile support which falls back to n == 0 in case Suite B profile
support is disabled, so n == -2 is the same as n == 0 in LibreSSL.

The API also returns 0 for error, which is indistinguishable from a
count of 0 shared groups but coincides with NID_undef. Contrary to claims
in the documentation, the API doesn't actually return -1 for clients,
rather it returns 0.

Obviously this entire exercise is pretty useless, but since somebody
exposed it because they could and someone else used it because they could
we need to provide it.

ok jsing

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